feature: tag tests; choose tags on command line

lib/src/backend/declarer.dart

Index: lib/src/backend/declarer.dart
diff --git a/lib/src/backend/declarer.dart b/lib/src/backend/declarer.dart
index 46a52d9e387da63ddbc9c6049445db6cc4239805..e337b15960f06ba5456d973ec3dbc60c94ae4897 100644
--- a/lib/src/backend/declarer.dart
+++ b/lib/src/backend/declarer.dart
@@ -73,11 +73,12 @@ class Declarer {
 
   /// Defines a test case with the given name and body.
   void test(String name, body(), {String testOn, Timeout timeout, skip,
-      Map<String, dynamic> onPlatform}) {
+      Map<String, dynamic> onPlatform, List<String> tags}) {
     _checkNotBuilt("test");
 
     var metadata = _metadata.merge(new Metadata.parse(
-        testOn: testOn, timeout: timeout, skip: skip, onPlatform: onPlatform));
+        testOn: testOn, timeout: timeout, skip: skip, onPlatform: onPlatform,
+        tags: tags));
 
     _entries.add(new LocalTest(_prefix(name), metadata, () {
       // TODO(nweiz): It might be useful to throw an error here if a test starts
@@ -94,11 +95,12 @@ class Declarer {
 
   /// Creates a group of tests.
   void group(String name, void body(), {String testOn, Timeout timeout, skip,
-      Map<String, dynamic> onPlatform}) {
+      Map<String, dynamic> onPlatform, List<String> tags}) {
     _checkNotBuilt("group");
 
     var metadata = _metadata.merge(new Metadata.parse(
-        testOn: testOn, timeout: timeout, skip: skip, onPlatform: onPlatform));
+        testOn: testOn, timeout: timeout, skip: skip, onPlatform: onPlatform,
+        tags: tags));
 
     // Don't load the tests for a skipped group.
     if (metadata.skip) {